1. Field of the Invention
The present invention relates to a hydraulic brake apparatus for supplying a hydraulic brake pressure to each wheel brake cylinder mounted on each wheel of a vehicle, and more particularly to the apparatus which is provided with a hydraulic pressure boosting device.
2. Description of the Related Arts
As for hydraulic brake apparatuses, various types are known and already on a market, including the apparatus of a type provided with a hydraulic pressure boosting device. For instance, the U.S. Pat. No. 3,928,970 discloses a power-assisted master cylinder using a source of fluid under pressure, which includes a piston movable in a cylinder bore, a pressure space ahead of the piston having an outlet for connection to a brake circuit, a power chamber behind the piston having an inlet for connection to the source of the fluid under pressure, and a control valve assembly for controlling the supply of fluid from the pressure source to the power chamber. It was the object of that patent to provide the assembly in which a piston is pedal operated but which does not require a movable inlet or inlet seals which move under pressure. And, there is disclosed a master cylinder assembly which includes a first piston and a second piston, between which a first pressure space is defined and connected to a hydraulic pressure circuit. Ahead of the second piston, a second pressure space is defined and connected to a space behind the first piston. An inlet valve member is provided for introducing the pressure discharged from the pressure source into the second pressure space, and an exhaust valve is provided for communicating the second pressure space with a reservoir, and these valve members are adapted to be actuated in response to movement of the second piston.
Also, the British Patent Application GB 2170874A discloses a servo-assisted master cylinder assembly for a vehicle hydraulic braking system which has a master cylinder with primary and secondary pistons operative to pressurize primary and secondary pressure spaces for primary and secondary brake circuits. A servo chamber is defined behind the primary piston, and pressurization of the servo chamber from a source of pressure fluid is controlled by valve means which comprises a spool operated solely by pressure in the primary space and a bias force from servo pressure and spring in a bias chamber. In the assembly disclosed in the application, the spool is arranged on a different axis from that of the master cylinder, i.e., in parallel with it, or perpendicularly to it.
According to the assemblies disclosed in the U.S. Patent and British Patent Application as described above, it is so arranged that the pressurized fluid which was regulated by a valve device is supplied to a power chamber (or, servo chamber). And, with respect to the regulating act done by the regulator, the valve device is adapted to control a balance between the regulated pressure and the master cylinder pressure, which are applied to the front and rear of a control piston, respectively. In operation, the hydraulic pressure of the pressurized fluid supplied from the source thereof is rapidly transmitted to the control piston, to make the pressure balance by repetition of increasing and decreasing the pressure of the fluid. Therefore, with the pressurized and regulated fluid supplied to the power chamber (servo chamber), a necessary boosting operation can be made. However, when a brake pedal is operated rapidly under the circumstance of a relatively low temperature for instance, the pressure decreasing operation might be initiated without sufficient amount of brake fluid supplied to the power chamber, so that sufficient boosting operation might not be made. In the case where the diameter of a passage for supplying the fluid to the power chamber is relatively small, or the length of the passage is relatively long, a proper response for boosting the pressure can not be obtained, so that conditions for designing the assembly will be severe.
According to the above-described assemblies, the regulated pressure discharged from the regulator is provided for various hydraulic brake pressure controls such as the anti-skid control. However, when the regulated pressure is provided for the hydraulic brake pressure control, the hydraulic pressure in the power chamber will be varied in response to variation of the regulated pressure. Then, the depressing force of the brake pedal will be varied in response to variation of the hydraulic pressure in the power chamber, whereby the brake pedal feeling to the vehicle driver might be deteriorated.
Furthermore, in order to increase the braking force when a rapid braking operation is made in case of emergency for example, Japanese Patent Laid-open Publication No. 9-24818 proposes a vehicle braking control apparatus, which includes a regulator for regulating power pressure discharged from a power source and supplying regulated pressure to wheel brake cylinders during a normal braking operation, and changeover means for communicating the pressure source directly with the wheel brake cylinders in case of an emergency braking operation. That publication discloses the regulator using a spool valve, and discloses means for varying an area for receiving the regulated pressure, so as to provide a desired characteristic of the regulated pressure to the master cylinder pressure. In practice, a resilient member such as rubber is placed ahead of the spool valve through an engaging member, to restrict the pressure applied to the spool valve, in accordance with increase of the area of the resilient member contacting with the engaging member. Also, Japanese Patent Laid-open Publication No. 9-24819 discloses an apparatus provided with an orifice for increasing the braking force in case of the emergency braking operation.
According to the above-described Japanese Patent Laid-open Publication Nos. 9-24818 and 9-24819, it is possible to increase the braking force in case of the emergency braking operation. In a transitional period of that braking operation, however, the braking force to de added may not necessarily be sufficient. Furthermore, when the brake pedal is released, the braking force will not be released immediately thereafter, so that the vehicle driver will be uncomfortable in his braking operation. In order to avoid this, it is necessary to provide a pressure sensor in the hydraulic pressure circuit to detect the release of the brake pedal. However, a relatively large delay occurs until the pressure decreasing operation will be initiated after the release of the brake pedal is detected. In stead of the expensive pressure sensor, it is preferable to employ a stroke sensor for detecting a stroke of the brake pedal. According to the apparatus disclosed in the publications described above, however, it is very difficult to detect the release of the brake pedal. The apparatus disclosed in the U.S. Pat. No. 3,928,970 is not adapted to increase the braking force in case of the emergency braking operation as described in the Japanese publications.